Microscopic particles, and especially pathogens, which lie in a stream of fluid such as air or water, can be detected by scattering of light (from infrared through visible to far ultraviolet, and possibly to soft x-rays). Usually a laser beam is employed as the light source because of its small size and high power density. The narrow laser beam is directed primarily perpendicular to the direction of fluid flow, and a plurality of photodetectors detect light scattered in different directions by a particle as it passes through the laser beam. The pattern of light scattering by an unknown particle can be compared to multiple scattering patterns for each of a plurality of known particles (which are pathogens). When there is a high correlation between the scatter subpattern, or eventvector for the unknown particle and multiple eventvectors for a particular known specie of particle, then this indicates a high probability that the unknown particle is the particular known specie of particle.
In constructing such a particle identifying device, it is important to assure that the laser beam has certain characteristics. It is desirable that only one particle at a time pass through the laser beam for accurate identification. It is also desirable that the period between successive particles passing through the laser beam is short, so that a large number of particles can be detected in a moderate period of time and during the passage of a sample of fluid that contains the particles. It is important that the laser be directed along a predetermined path and that it have a predetermined orientation and polarization. If the beam deviates considerably from the designated path, then photodetectors that pick up scattered radiation from a small detect zone along the path will not properly detect scattered radiation. Proper orientation of the polarized laser beam assures proper detection by photodetectors with polarizing filters and consequent correct identification of the particles.
In accordance with one embodiment of the present invention, an apparatus is provided for identifying microscopic particles in fluid by detecting scattering of light from particles passing through a detect zone lying along a narrow beam, which decreases the time spent by each particle intraversing the detect zone while increasing the frequency of particles passing through the detect zone. The beam has a width and thickness that are each perpendicular to the direction of the beam. The beam width, which is perpendicular to the direction of fluid and particle flow, is a plurality of times greater than the beam thickness which is parallel to the direction of fluid and particle flow. The small beam thickness results in each particle rapidly passing through the beam to minimize the number of times when two or more particles pass through the beam and data cannot be used. The wide beam width increases the frequency at which particles pass through the beam, to provide more data in a given time period.
The precise direction of the beam is checked in a test wherein the beam strikes a quadrant detector and the outputs of the quadrant detector are delivered to a difference circuit. The quadrant detector is of the type that has four photocells lying in each of four quadrants that are separated by perpendicular lines. In order to better detect the wide but small-thickness beam, the quadrant detector is oriented so the perpendicular lines are each angled about 45xc2x0 to the horizontal when the beam width direction is horizontal.
An activator is provided for moving the quadrant detector or beam relative to one another, in a direction parallel to the beam width. This enables a determination of the beam width at the quadrant detector. An activator also can move the quadrant detector relative to the beam in a direction parallel to the beam thickness to check the beam thickness.
The beam is generated by a laser diode whose output expands in both vertical and horizontal planes. A first primarily cylindrical convex lens in front of the laser diode, largely collimates the beam in a vertical plane to fix the thickness of the beam. A second primarily cylindrical convex lens which is on a side of a first lens opposite the diode, largely collimates the beam in a horizontal plane to fix the width of the beam. The spacing of the lenses is chosen to produce the desire ratio of width to thickness.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.